Servos
by Tom Carroll
I wrote about servos in this column back in the beginning of 2008 when the Robotis
Dynamixel rotary actuators had been available for a while for the robot experimenter. I want
to describe these, as well as the many other types that are for sale to robot builders. Those
little plastic boxes with the rotating plastic disc or X-shaped 'horn' have made small robot
design and construction easier for the beginner robot builder as well as for the experienced
experimenter. Rather than describing the history of servo mechanisms from the steam age to
the present, I will just concentrate on the typical servo that we might find in a hobby catalog
or in advertisements here in SERVO Magazine.

Those of us who have built robots over the years have always known
that robots do not have the strength
that the public ascribes to them. Steve
Austin — “The Six Million Dollar Man”
— might have been able to lift tons of
weight with his built-in man-made
muscles, and Robbie the robot in
Forbidden Planet (Figure 1) supposedly

FIGURE 1. Robby the Robot.

74 SERVO 11.2012

could topple a house off its foundation,
but we are actually quite happy if our
robots can lift a pound or two with
their arms. I happily watched this TV
show and movie (and many more that
featured robots), knowing full well that
‘real’ robots did not have the power
shown in the films.

Even human-sized industrial robots
lack the raw lifting strength of a person,
but make up for this inability with
extreme accuracy and the capability to
deliver many precision movements
without stopping.

Most servos of today certainly do
not have the strength that even small
animals the size of our robots might
have, but servos have given our smaller
creations amazingly complex
maneuverabilities.

Types of Servos

From just a handful of available
servos three decades ago, today’s
servos number in the many hundreds in
models, and millions are sold for all
sorts of applications. There are
numerous manufacturers, most of
which are from overseas. I am going to
review only a few manufacturer’s
products, such as Hitec who makes
over 50 different servos — about half
analog and half digital, with five of
those specifically for robots. Futaba has
over 130 in its line of servos, including
20 brushless and 11 in its line of S.bus
programmable servos.

Analog vs.

Digital Servos

For many years, everyone seemed
quite happy with the proportional
control offered by analog servos. Let’s
face it. These small, affordable servos
were first designed for model aircraft
radio control. Yes, some servos were
designed around radio control boat’s
and car’s needs, but the specifications
were mostly formed around aircraft
needs — especially small and light